Mac Mail locked

We have an Xserve running OS 10.5 server and acting as our mail server. One of our users complains that often he cannot send or receive emails because it says the account is locked. After some discussion with him, I found that he has his computer at home, his laptop, and iPhone all configured with his email account as a POP account. I advised him that he should have them setup as IMAP because of the "POP Lock" that the server puts on his account when one device accesses it. He still has not reconfigured his devices to use IMAP, but he is still complaining because he says that he will completely shut down two devices and the 3rd is still getting locked out. Is there something else I am missing? Another thing I noticed on his machine is that he has over 20,000 emails. I told him that he should really clean that up, but he refuses because I can't give him a "real answer" as to why he should. Oh, and this is the owner of the company so I can't use the "because your IT manager said to" or I'll get the "well I'm the owner so unless you can give me a good reason".

Thanks for your help.